1. Field of the Invention
The present invention relates to a method for estimating a thermal displacement of a rotatable spindle in a machine tool such as a machining center.
2. Description of the Background Art
A machine tool generally has heat sources at different parts thereof (e.g., roller bearing for a spindle) in the nature of the machine, and heat generated by the heat sources is transmitted to various parts of the machine, which causes a thermal displacement of the machine body. As the thermal displacement of the machine body largely affects machining accuracy, a method for cooling heat generating parts, or a method for estimating a thermal displacement from machine body temperature information for correction has been conventionally widely employed.
As the latter method for estimating a thermal displacement, the Patent document 1 disclosed a method where a thermal displacement is accurately estimated in any operational condition in such a way that a heat value during rotation is calculated based on an observed temperature changes during a period from a transient state after rotational speed is changed to a steady state, and then the thermal displacement is calculated from the calculated value.
Further, Patent document 2 discloses a thermal displacement estimating calculation where a thermal displacement of a spindle is obtained while varying a factor in a thermal displacement estimating arithmetic expression depending on time or the number of corrections, such that time response of temperature becomes the same as that of thermal displacement in a transient state, in order to make an accurate correction in any operational condition. In relation to the above method, Patent document 3 discloses the use of a function for making the time response of temperature equal to that of thermal displacement in order to cancel a dead time (delay) in temperature measurement caused depending on a time constant or mounted position (distance from a heat source) of a temperature sensor.
[Patent document 1] Japanese Unexamined Patent Publication No. 2006-15461
[Patent document 2] Japanese patent No. 3151655
[Patent document 3] Japanese patent No. 3422462